1. Field of the Invention
This invention relates to fabrication of printed circuit board assemblies and in particular printed circuit boards having through-hole components packaged in metal cases.
2. Description of the Related Art
Printed circuit boards for electronic circuits are widely used in almost every electronic field today. Printed circuit board assemblies are comprised of a printed circuit board and electronic components which are positioned on and electrically connected to circuitry formed on the printed circuit board. Printed circuit boards as manufactured have a continuous foil of conductive material adhered to at least one of the surfaces of a core of dielectric material. The continuous conductive foil on the printed circuit board is thereafter patterned using conventional techniques to remove portions of the conductive material so as to form a predetermined circuit pattern. Holes are punched or drilled through the circuit board at predetermined locations with the holes generally being in registration with selected solder pads formed as part of the circuit pattern etched in the conductive foil. Electronic components are then mounted at predetermined locations on the printed circuit board. The electronic components are temporarily secured in place until they can be permanently soldered in place.
Generally, when manufacturing printed circuit board assemblies leaded components, wherein the electronic components have leads extending from the casing for connecting the component to the circuitry of the board, the components are mounted on the surface of the board opposite the side where the circuitry is normally formed and the leads of the component are inserted through the preformed holes in the printed circuit board. The leads are then mass soldered to the circuitry of the printed circuit board by methods such as dip or wave soldering. Mass soldering of leaded components is a relatively straightforward process in that the leaded components are mounted on the top surface of the printed circuit board, which does not enter the solder bath, while the bottom surface having the circuitry defined thereon with the leads in registration with the solder pads is immersed in the solder bath for soldering.
The holes may also be plated through-holes which extend the full thickness of the board and open to solder pads on both surfaces on the board. Electronic components designed to mount to PCBs by means of plated through holes typically have depending metal leads which are spaced apart and sized to fit into corresponding plated through-holes and extend a small distance beyond the undersurface of the PCB. Conventionally, through-hole devices are connected to the PCB by machine wave soldering--i.e., the solder side of the circuit board is passed across a wave of molten solder. If two clean metal surfaces are held together and dipped into molten solder, the solder will wet the metal and climb up to fill the gaps between the adjacent surfaces. This phenomenon is the result of capillary action. Capillary action causes the molten solder to fill the annular space between the component lead and the walls of the plated through-hole. Upon cooling and solidifying, the solder forms a fillet around the lead and the ends of the plated through-hole.
There is a problem, however, with through-hole components packaged in metal cases. As the printed circuit board travels over the wave solder to solder components to the printed circuit board, solder wicks up the through-hole and shorts to the metal case on the top side of the board. Some components are designed with a stand-off feature built into the component itself in order to prevent the problem of shorting during soldering, however, many are designed to sit flush with the surface of the printed circuit board at the time of wave soldering. Industry practice solves this problem by installing an insulator (i.e. tape or any other pre-cut spacer) between the component and the board so that the component is raised from the printed circuit board surface and solder will not short to the component package. This method, however, requires additional material and labor to be added to the assembly process of the printed circuit board assembly in order to place the spacer underneath the component, therefore increasing manufacturing time and costs.